Analysis of Charge Distribution of lambda- and kappa-IgA in IgA Nephropathy by Focused Antigen Capture Immunoassay

Abstract

The finding that eluted mesangial IgA and serum IgA from patients with IgA nephropathy (IgAN) had a restricted anionic charge contrasting with normal serum IgA prompted us to examine the charge of κ- and λ-subclasses of IgA. Previous studies are not totally satisfactory because either total IgA without further dividing into subclass or only κ- and λ-lgA₁ was examined. A new approach is described to study the electrostatic property of total IgA and its light chain subclasses. The new focused antigen capture immunoassay (FACIA) allows us to separate the immunoglobulins by isoelectric point, then to capture by the heavy chain class and finally to visualize according to the light chain class. This method works well with whole blood or serum without the need of prior purification by affinity chromatography. The serum total IgA and λ-IgA levels in patients with IgAN were significantly higher than that of healthy controls. Elevated λ-IgA levels in patients with IgAN resulted in reduced κ/λ. ratio of total IgA. Similar to our previous findings in IgA₁ with O-linked oligosaccharide side chain, λ-IgA from patients with IgAN or health controls is highly anionic whereas λ-IgA is relatively cationic. The higher anionic/cationic ratio observed in total IgA from patients compared with controls was due to the higher concentration of λ-IgA from the former group. Raised anionic λ-IgA in IgAN may be contributory lo the immunopathogenesis through its selective mesangial binding.

Key Words:

• IgA
• IgA nephropathy
• isoelectric focusing
• focused antigen capture immunoassay